Scissors are commonly used to cut materials, such as paper, fabric, hair and the like. Scissors also come in a wide variety of sizes, from small scissors for cutting nails to a metal cutting scissors (e.g., shears).
Typically, scissors are constructed with two separate, slightly bowed blade members being pivotally coupled together by a pivot joint. The blade members are held at three main points: along the opposing cutting edge of each blade member, at the pivot joint, and by the contact between the blade members in back of the pivot joint and before the handle of the scissors. The pivot joint is placed under an axial load directed along the pivot axis of the pivot joint to keep the members together, while the contact in back of the pivot joint acts as a lever with the pivot joint as the fulcrum to produce tension and friction between the cutting edges of the blade members which ensures proper cutting action. There is also a corresponding friction or drag in typical prior art scissors between the blade members where they slide against each other at the point of contact in back of the pivot joint which is known in manufacturing as the "ride" or "half-moon." It is the combination of the pivot joint axial load with the lever contact in the "ride" area which determines the tension and friction along the cutting edges of typical prior art scissors.
Originally, the tension and friction in the scissors was non-adjustable. Typically, a threaded connecting pin with a pivot axis was passed through an oversized non-threaded hole in a movable blade member (with respect to the pin) and screwed into a threaded hole in the stationary blade member (with respect to the pin). The non-threaded pin end was enlarged to form a head or a bearing surface to press the opposing blade members against each other. The enlarged pin head served as the bearing surface for the pivotal movement of the moving member. The connecting pin could be adjusted slightly during manufacture to give slight variations in tension and friction. However, once manufactured, friction and tension in the scissors could not normally be adjusted by the user. Thus, the user was limited to the cutting tension and friction set by the manufacturer.
In non-adjustable scissors, the friction and tension changes over time from wear and loosening of the parts and by the accumulation of dirt and debris. As the parts wear and loosen, desirable tension and friction is reduced, thereby altering the alignment of the scissors. Misalignment causes poor cutting performance and efficiency, shortened tool life, as well as premature loss of edge sharpness. At the same time, undesirable friction or drag between moving parts greatly increases from a build up of dirt and debris between the pin head and the moving blade member, and between the opposing blade members where they make contact at the "ride" area. The result is impaired scissor movement or action due to excessive drag between moving parts.
In attempts to overcome these drawbacks, manufacturers have made the friction and tension in the scissors less sensitive to the effects of wear and the accumulation of dirt and debris. For example, either an anti-friction washer, bushing (usually nonmetallic), ball bearings, or sealed ball bearings have been interposed between the pin head and the moving blade member to reduce wear from friction. Threaded plastic bushings have been pressed into the threaded hole in the stationary blade member to accept the threaded pin and non-rotatively hold it, or the threaded pin is held in place by chemical thread-locking means (such as "Loctite thread locker") or by mechanical thread-locking means (such as deformable plastic strips, patch screws or lock nuts) to prevent wear on the threaded portion of the connecting pin and blade member. While these alternative designs may reduce wear in some parts, they do not eliminate wear along the cutting blades and wear at the "ride". Also, the alternative designs do not prevent or reduce the undesirable effects from the accumulation of dirt and debris between the moving parts and at the "ride" area.
In another alternative, thrust bearings have been interposed between the opposing blade members to reduce friction between the blade members. However, typical thrust bearings are relatively large and, thus, are limited to use on large scissors such as "pinking shears". Moreover, the large bearings cause the members to be widely separated, and thus the blades must exert a lever force on the rear most part of the thrust bearing, which extends into the "ride" area, to create the tension and friction in the cutting blades. This lever force produces wear with undesirable effects similar to that found in other typical prior art scissors. Also, the thrust bearings are especially prone to develop excessive drag through contamination by dirt and debris, because the thrust bearings are unsealed.
Typically, the above-described alternative designs do not provide for alteration of the tension and friction by the user. To allow adjustment of the tension and friction, as well as to address some of the above-described drawbacks, an adjustable tension positive-locking type pivot joint has been used. Typical scissors of this type are constructed like the non-adjustable scissors, except that the connecting pin is provided with either internal or external threads, to which a locking screw or nut is affixed for engaging the opposing blade members together with varying pivot axial loads to adjust the tension and friction. In some scissors, the locking screw or nut is user adjustable, thereby allowing for tailoring of the friction and tension to fit the needs of the individual user.
However, while this type of scissors has adjustable tension and friction, it still suffers from several drawbacks. The operator-adjustable pivot joint may be large and bulky so that it interferes when the scissors are used with another device, such as a guide, a comb or the like. Moreover, frequent adjustment of the adjustable pivot joint may be required to compensate for the locking screw or nut loosening rotationally due to an inadequate locking force (i.e., caused by wear or by poor design) or unintentional contact with the operators hand, or other object, while in use. Also, like in the previously described scissors, continual adjustment of the adjustable pivot joint is required to compensate for loosening blade member tension from wear of sliding parts. Moreover, adjustments of the adjustable pivot joint may be required to compensate for the increased friction or drag between other moving parts from the collection of dirt, debris and corrosion. Typically this accumulation occurs between the pin head and the moving blade member, and between the opposing blade members where they make contact at the "ride".
Thus, even with tension adjustable scissors, the operator is distracted from efficient cutting by the intrusive protrusion of the tension adjusting pivot joint, and the necessity of adjusting the blade member tension to compensate for wear or the loosening of the adjustable pivot joint itself. Tension adjustable scissors give the user greater control over tension and friction, but they do not reduce effects of wear and accumulation of dirt and debris. Therefore, the wear in tension adjustable scissors still results in poor cutting performance and efficiency, shortened tool life, and loss of cutting edge sharpness.